Method for the direct reduction of iron oxides

ABSTRACT

Method for the direct reduction of mineral iron inside a vertical reduction furnace (10) of the type with a gravitational load, wherein the reduction gas flows in counter-flow with respect to the material introduced into the furnace, comprising the following steps: the mineral iron is fed from above into the furnace (10), a mixture of high temperature gas consisting of reducing gas based on H2 and CO is injected, and the reduced mineral is removed from the furnace (10), the mixture of gas being introduced in at least two zones (12, 14) of the furnace (10) arranged one above the other so as to achieve, in a controlled manner, a first stage of pre-heating and pre-reduction in the upper part (12) of the furnace (10) and a second stage of final reduction in the lower part (14) of the furnace (10).

FIELD OF THE INVENTION

This invention concerns a process to produce metallic iron starting frommineral iron, wherein the iron is present in the form of oxides, and therelative apparatus which comprises a reduction furnace with multipleinlets for the reducing gas and inside which the process of directreduction of the iron (DRI) is carried out. The reduced iron may exitfrom the reduction furnace either hot or cold and may be subsequentlysent to a melting furnace to produce liquid steel, or converted into hotbriquette steel (HBI).

BACKGROUND OF THE INVENTION

The state of the art includes processes of direct reduction which usethe injection of hydrocarbons into the current of reducing gas to allowthe reaction of reforming the methane in the furnace with the H₂O andCO₂ in the gas; there are also known processes of direct reduction whichuse the injection of hydrocarbons with C>5 directly into the furnace inthe zone between the injection of the reducing gas and the outlet fromabove of the burnt gas.

From the following patent documents other processes are known for thedirect reduction of mineral iron:

U.S. Pat. Nos. 2,189,260, 3,601,381, 3,748,120, 3,749,386, 3,764,123,3,770,421, 4,054,444, 4,173,465, 4,188,022, 4,234,169, 4,201,571,4,270,739, 4,374,585, 4,528,030, 4,556,417, 4,720,299, 4,900,356,5,064,467, 5,078,788, 5,387,274, and 5,407,460.

The state of the art also includes processes wherein the hot metalliciron is produced in a reduction furnace of the shaft type, with avertical and gravitational flow of the material, which is subsequentlysent to the melting furnace by means of a closed pneumatic transportsystem in an inert atmosphere.

SUMMARY OF THE INVENTION

The method to produce metallic iron by the direct reduction of ironoxides and the relative apparatus according to the invention are setforth and characterised in the respective main claims, while thedependent claims describe other innovative features of the invention.

The method according to the invention consists in bringing into contactthe mineral iron, of various granulometry, with a feed gas in areduction furnace of the shaft type, wherein both the gas and thematerial are fed continuously, so that a vertical and gravitational flowof material is created and the direct reduction of the mineral isachieved. The material may be discharged from the reactor either cold orpreferably hot to be sent subsequently to a melting furnace or so thatit may be converted into hot briquette iron (HBI) or cooled andconverted into direct reduction iron (DRI).

The reduction furnace is equipped with means to feed the mineral ironand means to discharge the reduced metallic iron; it is equipped with atleast two inlet collectors to inject the reducing gas in correspondencewith different reduction zones inside the furnace to ensure a greaterreduction zone.

The reduction gas sent into the reactor contains hydrocarbons injectedinto the current after the partial combustion of the hydrogen and carbonmonoxide with the oxygen.

In a variant, the hydrocarbons are injected before the partialcombustion is achieved, with the purpose of raising the temperature ofthe gas introduced into the reactor.

According to another variant, the hydrocarbons are at least partlyinjected into a zone between the reduction zone and the zone where thereduced material is discharged.

In all cases, the injected hydrocarbons cooperate in reducing the ironoxide (FeO) to metallic iron, generating more H₂ and CO.

The direct reduction of the iron oxides is achieved in two differentcontinuous stages inside the reduction reactor.

In a first stage, defined as the pre-heating and pre-reduction stage,the fresh iron oxides, that is, those just introduced into the furnace,come into contact with a mixture of reduction gas, consisting of partlyburnt gas, arriving from the underlying part of the furnace and of freshhot gas, that is, gas introduced from outside, arriving from a collectorwhich brings fresh reducing gas and possibly CH₄ or other natural gas.This first stage takes place in a corresponding first zone arranged inthe upper part of the furnace.

In the second stage, or real reduction stage, the complete reduction ofthe iron oxides is achieved, due to the action on the oxides, alreadypartly reduced in the first stage, of a mixture of reducing gas based onH₂ and CO and at least a hydrocarbon, preferably natural gas, injectedin the median zone of the reduction reactor. This second stage takesplace in a corresponding second zone arranged below the first zone.

The two inlets to the furnace, through which the gas is introduced, canbe independently regulated both in the flow of fresh reducing gas and inthe addition of natural gas in the current introduced.

Moreover, the inlet temperature of the two currents of reducing gas canbe independently regulated by injecting O₂ before they enter thereduction reactor.

The oxidation reaction needed to raise the temperature of the gas leadsto a change in the level of oxidation of the gas, from normal values of0.04-0.08 to 0.06-0.15.

The following ratio is intended for the level of oxidation of thereducing gas:

Nox=(H₂O+CO₂)/(H₂O+CO₂+H₂+CO)

In the second reaction zone of the furnace, wherein the reduction of theiron oxides is completed, a gas is generated with a high content of H₂and CO and with an oxidation level of between 0.15 and 0.25 due to thereduction reactions of the iron oxides with H₂, CO and CH₄.

Once this gas has left the second reaction zone, it enters the firstreaction zone, located higher up, and mixes with the hot gas injectedinto the first zone to pre-heat and pre-reduce the iron oxides.

The gas emerging from the reduction reactor is partly recircled andpartly used as fuel.

The recircled gas has a volume composition within the followingfields:H₂=20-41%, CO=15-28%, CO₂=15-25%, CH₄=3-10%, N₂=0-8%, H₂O=2-7%.

According to one characteristic of the invention, the gas feeding thereduction reactor consists of a mixture of natural gas, recircled gasfrom the reactor itself and reformed gas; the recircled gas ispre-heated to a temperature of between 650° C. and 950° C.; the gasemerging from the pre-heater is in turn mixed with fresh reformed gasand subsequently with air, or air enriched with oxygen, or pure oxygen,to carry out a partial combustion of the H₂ and CO in the reducing gasin order to raise the temperature to values of between 800° C. and 1150°C., preferably between 10000° C. and 11500° C.; and the oxidation levelof the resulting gas feeding the furnace is between 0.06 and 0.15.

The methane represents between 6 and 20% in volume of the mixture ofreducing gas.

When the feed gas comes into contact in the reduction zone with the hot,partly reduced material, which therefore consists partly of metalliciron and partly of iron oxides, a highly endothermic reaction isproduced.

There is a also an endothermic reaction in the pre-heating andpre-reducing zone when the gas comes into contact with the iron oxide.

One advantage of this invention is that the first pre-heating andpre-reducing zone is extended, which allows to start the transformationof the Ematite (Fe₂O₃) into Wustite (FeO) more quickly.

The whole reactor works at a higher average temperature and above allwhich is constant along both zones, both the pre-reduction and reductionzones, encouraging a higher reaction speed, with a consequent effect ofreducing consumption and increasing productivity.

The first inlet for the reducing gas is located at a set distance (x)with respect to the second inlet, which is located in the median part ofthe furnace, in correspondence with the second reduction zone. Thisdistance (x) is suitably included between 1 and 6 meters, preferablybetween 2 and 4 meters, to encourage the reactions in the most suitablezone between the reducing gas and the iron oxides.

The first gas inlet also has the function of pushing the gases arrivingfrom the second reduction zone towards the centre of the furnace so asto create a uniform distribution of the gas in the section of thereactor.

According to a variant, there are multiple inlets for the reducing gasinto the furnace, or more than two. The first current of reducing gas isinjected into the middle of the reactor, into the reduction zone proper,while the other currents are introduced into the zone between theinjection of the first current of gas and the outlet of the burnt gas,in the upper part of the furnace. This intermediate zone will be calledthe pre-heating and pre-reducing zone for the iron oxide based material.

The flow of gas into the reactor thus composed allows to have the wholereduction and pre-reduction zone at as constant a temperature aspossible, and to have a gas inside the furnace which always has a highreducing power, encouraging a greater productivity and a lowerconsumption of gas; this also allows to improve the final metalisationof the product.

In this way, moreover, the iron oxides arrive at the reduction zonealready partly reduced, thus encouraging the completion of the finalreduction reaction from FeO to Fe.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other characteristics of the invention will become clear fromthe following description of a preferred form of embodiment, given as anon-restrictive example with the aid of the attached Figures wherein:

FIG. 1 shows in diagram form an apparatus for the direct reduction ofiron oxides according to the invention;

FIG. 2 is a first variant of a furnace employed in the apparatus in FIG.1;

FIG. 3 is a diagram showing the temperature inside the furnace shown inFIGS. 1 and 2;

FIG. 4 shows a second variant of a furnace employed in the apparatus inFIG. 1;

FIG. 5 is a diagram showing the temperature inside the furnace shown inFIG. 4; and

FIG. 6 shows another variant of the apparatus in FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to FIG. 1, an apparatus for the direct reduction of ironoxides according to the invention comprises a reduction furnace of theshaft type or reduction reactor 10, comprising in turn an upper mouth 11for feeding from above, through which the mineral (iron oxides) is ableto be introduced, a first pre-heating and pre-reduction zone 12, asecond zone, or median zone 14 wherein the final reduction reaction ofthe iron oxides takes place, and a lower zone, or discharge zone 15,shaped like a truncated cone, terminating at the bottom in a loweraperture 16 through which the iron is discharged.

The iron-based metal oxides are introduced into the reactor 10 in theform of pellets or crude mineral in the appropriate sizes; the ironcontained therein is usually between 63% and 68% in weight.

At the end of the process according to the invention, the iron containedin the reduced material exiting from the reactor 10 is normally between80% and 90% in weight.

In correspondence with the two zones 12 and 14 of the reactor 10 thereare two independent inlets 17, respectively 18, through which a mixtureof gas is suitable to be introduced, as will be described in greaterdetail hereafter.

In its upper part, above zone 12, the reactor 10 is provided with anaperture 19 through which the burnt gas exits. This gas normally has thefollowing characteristics: composition: H₂=20-41%, CO=15-28%,CO₂=12-25%, CH₄=2-10%, N₂=0-8%, H₂O=2-15%; temperature between 500° C.and 700°C.; oxidation level between 0.3 and 0.50, preferably between0.40 and 0.45; and a reduction ratio R of between 1 and 1.8 wherereduction ratio is taken as:

R=(H₂+CO)/(H₂O+CO₂)

The burnt gas emerging from the reactor 10 is sent through a pipe 20 toa cooling unit 21, suitable to recover the heat which can be given up;from the cooling unit 21, through another pipe 22, it arrives at acooling and condensing unit 24. In this unit 24 the burnt gas is washedin water at a temperature of between 40° C. and 65° C. and the quantityof water present in the gas itself is partly removed. The percentage ofwater remaining in the gas at outlet from the unit 24 is between 2% and7%.

The gas at outlet from the unit 24 is sent through a pipe 30 partly to apre-heater 36, partly to a catalytic reformer 44, to be used as fuel,and partly to a compressor 26.

The gas emerging from the compressor 26 is in turn used partly as arecircling gas and sent, through a pipe 28, inside the unit 21, andpartly, through a pipe 46, mixed with a natural gas comprising methane(CH₄), or pure methane, arriving from a pipe 34 in a proportion of about4:1 (that is to say, for every part of natural gas there are about fourparts of gas arriving from the pipe 46) and introduced into the reformer44 so that the reforming reaction of the methane (CH₄) with H₂O and CO₂can begin.

The part of gas which is sent to the unit 21 through the pipe 28 ispre-heated, and is then sent through a pipe 32 to the pre-heater 36,where it is further pre-heated to a temperature of between 650° C. and950° C. CH₄ may also be injected into the pipe 32.

The gas emerging from the pre-heater 36, which has a delivery rate ofbetween 600 Nm³/ton DRI and 1500 Nm³/ton DRI, is mixed in a pipe 38 withthe gas arriving from the reformer 44 through a pipe 50.

The gas resulting from this mixture is divided into two parts anddistributed into two pipes 40 and 41, connected to the inlets 17 and 18of the furnace 10. The delivery of reducing gas is controlled in eachzone 12, 14 by means of regulation valves 55 and 56.

Into each pipe 40 and 41 air is injected, or air enriched with oxygen orpure oxygen and natural gas in variable percentages, in order to achievea partial combustion of the CO and the H₂ and raise the temperature ofthe gas.

A current of CH₄ or natural gas is injected into the gas before it isintroduced into the reactor.

In a variant, shown by a line of dashes in FIG. 1, the CH₄ is injectedbefore achieving the partial combustion, with the purpose of raising thetemperature of the gas introduced into the reactor.

The CH₄ may also be introduced in a zone between the reduction zone 14and the discharge cone of the material, through a pipe 81. In this case,before entering into the zone 14 where the reduction reactions arecarried out, the CH₄ injected partially cools the reduced iron, beforethe latter is discharged.

Valves V1-V11 are located in correspondence with the different conduitsof the plant so that the flow can be selectively controlled.

The resulting mixtures are introduced into the pre-heating andpre-reduction zone 12 and respectively into the reduction zone 14.Therefore, for each zone 12 and 14 the corresponding mixture of gas isregulated in an autonomous and independent manner.

To be more exact, the flow of gas in the first zone 12 is between 500Nm³/ton DRI and 800 Nm³/ton DRI and enters the reduction reactor 10 witha temperature of between 800° C. and 1150° C., preferably between 1000°C. and 1150° C., while the flow of gas in the second zone 14 is between1000 Nm³/ton DRI and 1500 Nm³/ton DRI and also enters the reductionreactor 10 with a temperature of between 800° C. and 1150° C.,preferably between 1000° C. and 1150° C.

The consumption of oxygen, which is necessary to raise the temperatureof the reducing gas from 650° C.-950° C. to 800° C.-1150° C., intendedas pure oxygen plus that contained in the air, if air is also injected,is between 8 Nm³/ton DRI and 60 Nm³/ton DRI, preferably between 20 and60 Nm³/ton DRI.

The consumption of CH₄ is between 50 and 120 Nm³/ton DRI, preferablybetween 90 and 110 Nm³/ton DRI.

In volume the CH₄ represents between 6 and 20% of the mixture ofreducing gas introduced into the reactor.

The reactions involved in the reduction zone 14 are as follows;

FeO+CH₄=Fe+2H₂+CO  (1)

Simultaneously, in the same zone 14, the following reduction reactionstake place with hydrogen and carbon monoxide:

FeO+H₂=Fe+H₂O  (2)

FeO+CO=Fe+CO₂  (3)

The consequence of these endothermic reactions is that the temperatureof the gas in the reduction zone decreases from 800° C.-1150° C. to 700°C.-900° C., yet still maintains the reaction temperature higher than infurnaces in the state of the art, and the gas leaving the reduction zone14 has an oxidation level of between 0.15 and 0.35 and a reducing powerof between 1.1 and 2.8.

The reactions involved in the pre-reduction zone 12 are as follows:

Fe₂O₃+H₂=2FeO+H₂O  (4)

Fe₂O₃+CO=2FeO+CO₂  (5)

In the lower zone 15, shaped like a truncated cone, it is also possibleto introduce gas containing natural gas to control the final carbon inthe hot reduced iron to values of between 1.5% and 3.0%.

In a first variant as shown in FIG. 2, instead of having a single lowerpart shaped like a truncated cone, the furnace 10 has at least two, andpreferably three or four lower ends, shaped like a cone or truncatedcone 15 a, 15 b and 15 c, through which the reduced metallic iron isdischarged in a controlled and independent manner. In this case the CH₄may also be introduced by means of devices located on the zone ofintersection of the truncated cone ends 15 a, 15 b and 15 c, thusexploiting the geometric conformation of the system.

The development of the temperature inside the furnace 10, both in theversion shown in FIG. 1 and also in the variant shown in FIG. 2, isshown in FIG. 3, from which it can be seen how the temperature is higherand more constant in the segment affected by the two zones 12 and 14.

According to a second variant shown in FIG. 4, instead of having twoinlets to introduce reducing gas, the furnace 10 is provided with aplurality of inlets, more than two. In this case a first current of gasis introduced into the lower inlet 18 through the pipe 41, a secondcurrent of gas is introduced into the inlet 17 through the pipe 40 andother currents of gas, each of which can be autonomously regulated, areintroduced through pipes 42 and corresponding inlets 43 arranged betweenthe inlet 17 and the upper aperture 19.

The development of the temperature inside the furnace 10, in the variantshown in FIG. 4, is shown in the diagram in FIG. 5, from which it can beseen how the temperature is higher and more constant in the wholesegment affected by the pipes 40, 41 and 42.

According to another variant, shown in FIG. 6, the reducing processinggas may be recircled without passing through a catalytic reformer, but apart of the gas exiting from the reduction furnace 10 is pre-heated inthe exchanger 21 and, by means of the pipe 32, mixed with natural gas,for example CH₄, and sent to the pre-heater 36.

In this variant, the gas exiting the furnace 10 has a temperature ofbetween 500° C. and 600° C. and has the following composition:H₂=30-36%, CO=20-25%, CO₂=20-25%, CH₄=2-7%, H₂O=15-25%; with anoxidation level of between 0.4 and 0.5.

The gas, thus pre-heated and mixed with natural gas, exits thepre-heater 36 at a temperature of between 650° C. and 950° C., it issubsequently divided into several currents of reducing gas, into each ofwhich oxygen and natural gas are injected before they enter thereduction furnace 10, so as to raise the temperature of the inlet gasesto a value of between 800° C. and 1150° C.

Another part of the gas exiting the reduction furnace 10 is used as fuelto generate heat in the pre-heater 36, by means of the pipe 30.

The reactions which take place in the reduction furnace 10 are topre-heat and pre-reduce the mineral in the upper zone 12 and to reducethe Wustite (FeO) with CH4, H2 and CO in the reduction zone 14.

In a variant, CH₄ may be injected into the zone between the reductionzone 14 and the truncated-cone-shaped discharge end 15; in this way theCH₄ is pre-heated, cools the reduced material, and arrives in thereduction zone 14 cooperating with the methane contained in thereduction gas injected in the reaction zone 14.

With this system it is possible to eliminate the catalytic reformer 44,and at the same time the plurality of gas inlets allows to improve theprofile of the temperature of the reduction furnace 10, making it moreuniform and accelerating the reduction reactions.

Obviously, it is possible to make modifications and additions to themethod for direct reduction of mineral iron and the relative apparatusas described heretofore, but these will remain within the field andscope of the invention.

What is claimed is:
 1. Method for the direct reduction of mineral ironinside a vertical reduction furnace with a gravitational load, whereinthe reduction gas flows in counter-flow with respect to the materialintroduced into the furnace, comprising the following steps: feedingmineral iron from above into the furnace, injecting a controlled amountof a first mixture of high temperature gas in a first reducing zone ofsaid furnace, injecting a controlled amount of a second mixture of hightemperature gas in a second zone of said furnace, said first and secondzones being arranged one above the other, removing burnt gas from anupper part of said furnace, and removing reduced mineral from a lowerpart of said furnace, preheating at least a part of said burnt gas to atemperature between about 650° and 950°, mixing the preheated burnt gaswith a reducing gas based on H₂ and CO, in order to produce a mixed gas,and injecting a controlled quantity of at least a hydrocarbon and oxygeninto a first part of said mixed gas, so as to increase the temperatureof said first part of said mixed gas between about 800° and 1150° C. andform said first mixture of high temperature gas, and injecting acontrolled quantity of at least a hydrocarbon and oxygen into a secondpart of said mixed gas, so as to increase the temperature of said secondpart of said mixed gas between about 800° and 1150° C. and form saidsecond mixture of high temperature gas.
 2. Method as in claim 1,characterized in that said hydrocarbon comprises methane.
 3. Method asin claim 1, characterized in that the amount of said first mixture ofhigh temperature gas injected in said first zone is different than theamount of said second mixture of high temperature gas injected into saidsecond zone.
 4. Method as in claim 1, characterized in that saidhydrocarbon comprises natural gas and in that the percentage of saidhydrocarbon in said first mixture of high temperature gas is controlledindependently of the percentage of said hydrocarbon in said secondmixture of high temperature gas.
 5. Method as in claim 1, characterizedin that said first mixture of high temperature gas is heatedindependently of said second mixture of high temperature gas beforebeing injected in each one of said first and second zones along thevertical length of said furnace.
 6. Method as in claim 1, characterizedin that said reducing gas comprises a mixture of a variable andcontrolled percentage of said burnt gas and of other gases.
 7. Method asin claim 1, characterized in that said reducing gas comprises a mixtureof a variable and controlled percentage of said burnt gas and of gasarriving from an outside catalytic reformer.
 8. Method as in claim 1,characterized in that said mixture of high temperature gas injected intosaid reactor has an oxidation level of between 0.06 and 0.25.
 9. Methodas in claim 1, characterized in that further CH₄ is partly injected intosaid furnace in a zone between said lower part and an underlyingdischarge zone.